1. Field of the Invention
The present invention relates to an optical filter and an optical instrument. Priority is claimed on Japanese Patent Application No. 2003-084984, filed Mar. 26, 2003, and Japanese Patent Application No. 2003-299223, filed Aug. 22, 2003 the contents of which are incorporated herein by reference.
2. Description of Related Art
A fluorescence microscope, which is an optical instrument used for observing biological specimens, analyzes the structure and nature of a specimen, such as a cell that has been treated with dye, by observing fluorescent light emitted by the specimen when excitation light is irradiated thereon.
In order to perform the latest genomic analysis, there is a need to observe, for example, both fluorescent light having a peak at 526 nm and excitation light having a wavelength of 502 nm. In this case, because the wavelength of the excitation light is close to the wavelength of the fluorescent light, in order for the fluorescent light to be more efficiently detected, an optical filter that cuts out the excitation light using a stopband and that allows light of the fluorescent light observation wavelength to pass through using a transmission band is used as an extremely important key part in order to determine the sensitivity and accuracy of the fluorescent light measurement.
In this optical filter, properties that permit a sharp rise in the spectral characteristics at boundaries between transmission bands and stopbands, and that also allow substantially 100% of light to be transmitted in the transmission band are required. Furthermore, in the transmission band, it is desirable that there are no cyclic variations (i.e., ripples) in the transmittance in response to increases or decreases in the wavelength.
A minus filter, which is an optical filter that cuts out light in a predetermined wavelength band and allows light of other wavelengths to pass through in this manner, is manufactured, as is shown in FIG. 10A, using a multi-layer film in which layers having a high refractive index and layers having a low refractive index are laminated alternately on a substrate. In FIG. 10A, the horizontal axis shows the optical thickness while the vertical axis shows the film refractive index. In addition, FIG. 10B is a graph in which the relationship between the transmittance and the wavelength of light that passes through a film during construction of the film is shown as a spectral characteristic.
The optical filter is able to make the rise at boundaries between transmission bands and stopbands sharper as the number of the aforementioned layers is increased. Moreover, as is shown in FIG. 11A, it is also possible to design a film in which ripples are reduced by changing the optical thickness of each layer. FIG. 11B shows a case in which the ripples have been reduced.
As is shown in FIG. 12A, if the refractive index of the film is changed cyclically and continuously in the film thickness direction such that the refractive index distribution thereof is formed into what is known as a “wavelet” configuration, then, as is shown in FIG. 12B, it is possible to fundamentally cancel ripples in the transmission band (see, for example, Non-Patent Document 1—“Using Apodization Functions to Reduce Sidelobes in Rugate Filters”, W. H. Southwell, Applied Optics, Vol. 28 (1989) Pp. 5091–5094).
For example, various types of structures have been proposed such as those in which a continuous refractive index distribution is divided into steps and approximated, as is shown in FIG. 13A, and structures in which a repeating layer is provided in which the respective refractive indices of a high refractive index layer and a low refractive index layer in an intermediate portion of a cycle are uniform (see, for example, FIG. 1 in Japanese Patent No. 3290629, and Non-Patent Document 2—“Synthesis of High Rejection Filters with the Fourier Transform Method”, P. G. Very, J. A. Dobrowlski, W. J. Wild, and R. L. Burton, Applied Optics, Vol. 28 (1989) Pp. 2864–2875, and Non-Patent Document 3—“Handbook of Optics, Second Edition, Vol. 1, Fundamentals, Techniques, and Design”, Optical Society of America, published by McGraw-Hill, 1995, Pp 42, 50).